I. Field of the Invention
The present invention relates to wireless network design. More particularly, the present invention relates to a method of simulating and analyzing the operation of a code division multiple access (CDMA) wireless telecommunications network.
II. General Background of the Invention
Network planning requires an estimation of system requirements in light of expected future load characteristics. A computer based network planning tool is one convenient means of network planning. The network planning tools for analog and digital wireless frequency division communication systems are often quite different from planning tools used for spread spectrum based communication systems. Frequency division systems, such as implemented in time division multiple access (TDMA), frequency division multiple access (FDMA) and automatic mobile phone service (AMPS), need sophisticated frequency planning methods. A typical frequency planning method used in these systems will ensure that each individual user is given a distinct frequency when communicating through the same base station receiver unit. In addition, a frequency assigned to one user on a first base station can only be reused on a nearby base station if it does not interfere with neighboring base stations transmission requirements.
More sophisticated network planning tools are required when the underlying communication system is more complex. Sophisticated spread-spectrum digital communication systems, like CDMA, have increased the quality of communication as well as increased the complexity of the network. Unlike planning tools used for the previously mentioned digital and analog, spread spectrum systems have very complex analysis requirements. A spread spectrum system utilizing a wide-band communication scheme, like CDMA, will require specialized planning tools which take into account transmission characteristics related to analog signals as well as the various digital modulation methods implemented. A brief comparison of a spread spectrum system, like CDMA, with other traditional wireless communication methods should make the increased level of complexity apparent.
Unlike a spread spectrum communication, the traditional multiple access system utilizes exclusive resource allocation methods. An exemplary exclusive resource allocation method will provide a frequency or a time slot, or a combination thereof, which is disjoint from those of any other user. This method may be found in analog, such as AMPS and digital, such as TDMA, wireless communication methods. Assuming each transmission occurs in perfect isolation from all the other users, the multiple access channel may be typified as a multiplicity of single point-to-point channels. The capacity of each of these point-to-point channels usually is limited only by the bandwidth, noise degradation, multi-path fading and shadowing effects.
The bulk of traditional network planning methods, therefore, is concerned with estimating the likelihood these point-to-point channels will be available. The availability of a channel at any moment, in turn, depends primarily on limitations of frequency reuse. Frequency reuse requires a network planner to allocate a finite number of frequencies amongst base stations as associated coverage areas in such a manner that reused frequencies are out of range of other nearby cells. The traditional network planning tool only needs to estimate signal loss to determine if a proper network constellation will function. The analysis is complete once the planning tool has determined all coverage areas transmitting at the same frequency are far enough apart that signal interference will not occur.
A hard capacity limit also restricts users in a traditional system due to an absolute limit of frequencies available. A subscriber unit attempting to communicate over an analog system, such as AMPS, or particular digital systems, such as TDMA, faces hard capacity limit. This hard limit depends on the number of frequencies available at a particular coverage area. A typical analog system has fifty seven analog FM channels in a three-sector coverage area. When demand for service is at a peak, the fifty-eighth caller in a given cell must be given a busy signal. Similarly, when a call in progress must hand-off and all adjacent coverage areas are full this subscriber call will likely be dropped when the signal strength waxes and the no handoff is possible.
The analysis methods found in a traditional network planning tool are static despite the dynamic nature of communication. These methods can not take in account a sudden increase in users or interference caused by unexpected electromagnetic transmissions. In practice, when these unaccounted for events occur a traditional network will respond by denying users service and a loss in signal quality. Clearly, the simplicity of such traditional systems and their respective planning tools has been obtained at the expense of system integrity. As will be seen, wideband wireless systems, like CDMA, take in account many dynamic factors and as a result have much more complex methods of analysis.
In a wideband system, like CDMA, there is a much softer relationship between the number of users and the grade of service. The grade of service (GOS) is an index which measures the likelihood a call will be dropped or service refused. Variation of the GOS in a wideband system may vary in proportion to the number of users but is not bounded by the hard capacity limits found in narrow-band systems. This is because the wideband system allocates all resources to all users simultaneously. Available bandwidth is maximized using sophisticated coding techniques which provide a resilient transmission medium impervious to cross-talk caused in traditional wireless methods previously mentioned.
The capacity of a wideband system is limited only by the ability to decode the transmitted signal. Each user communicating sends an message encrypted with a pseudo-random code which is decrypted by the receiving base station. As the number of users increase at a base station, the probability that the receiving base station will decrypt the code accurately decreases. A system operator could decide to allow a small degradation in the bit error rate and increase the number of available channels during peak hours. Therefore, a spread spectrum communication planning tool must determine the serviceable number of users depending on the quality of transmission selected, the projected number of simultaneous users and the corresponding interference during the decoding stages.
Power consumption and control is another unique area of planning required by a spread spectrum system. To achieve high capacity, the wideband system employs power control methods on both the forward link, from a base station to a subscriber unit, and reverse link, from a subscriber unit to a base station. The objective of the reverse link power control process is to require that each base station need only receive nominal signal power. When all the subscriber station transmitters within the coverage of a base station are so controlled, then subscriber unit energy is conserved and less noise and interference results. The forward link power, though practically not finite, must be adjusted to include only those subscriber units which are capable of returning a like signal. Matching the forward link power with the nominal reverse link power insures a complete communication link from the forward link and back over the reverse link is possible. A planning tool for a wideband communication system must be able to determine numerous system power levels as a function of dynamic variables like user load and signal interference.